Conventionally, in a lens barrel using a glass lens, a lens has been fixed to a lens barrel by crimping a lens holding portion on the lens barrel side, which is made of a molding material. This case has the following problems.
In other words, there are listed problems in that 1. the glass lens is expensive in view of component costs, 2. the glass lens is heavy in weight, and 3. the glass lens has no freedom in shape and thus it is impossible to form a crimp shape such as a boss and a hole. Accordingly, there is no choice but to caulk the lens barrel side. The glass lens is required to have space for crimping outside an outer diameter dimension required optically, by which the weight becomes heavy.
Meanwhile, in a lens barrel in which a plastic lens is attached to a lens holding portion, an attaching method of pushing the plastic lens to the lens barrel by a spring force using a plate spring is disclosed (for example, refer to Patent Document 1).
However, this case has the following problems. In other words, the number of components and the costs corresponding to the plate spring are required. Furthermore, an optimal fixing pressure that does not affect lens surface accuracy needs to be set.
Here, a concrete explanation of thermal crimping using a conventional plastic lens according to Patent Document 2 is illustrated in FIGS. 6 and 7. FIG. 6 is a cross-sectional view showing an outline shape of the plastic lens and FIG. 7 is a front view showing an outline shape of a lens holding portion.
In this lens barrel, a plastic lens 3 is crimped in an attached state to a lens holding portion 2 and fixed. As shown in FIG. 6, as a material of the plastic lens 3, an epoxy-based thermoplastic resin (for example, ZEONEX 480S: trade name, produced by Zeon Corporation) is used.
In the circumference of the lens 3, three fixed protrusions (bosses) 4, 5, 6 each having a thickness in a range of φ1 mm−0.01 mm to −0.03 mm are provided. In contrast, in the lens holding portion 2, three supporting holes, that is, a reference hole 8, a loosely fitting hole 9, and an elongated hole 10 are provided, as shown in FIG. 7.
The reference hole 8 is formed with φ1 mm+0.01 mm to 0, which is almost the same size of the fixed protrusion 4 of the plastic lens 3, and fits with the fixed protrusion 4 to thereby be positioned.
The loosely fitting hole 9 is formed with φ1.2 mm+0.03 mm to +0.01 mm, which is formed so as to be larger than the fixed protrusion 5 of the plastic lens 3, and has looseness in a plane direction in a state in which the fixed protrusion 5 is fitted.
The elongated hole 10 is formed with (φ1 mm+0.015 mm to 0)×1.2, and the fixed protrusion 6 of the plastic lens 3 is fitted into the elongated hole 10 to thereby position the lens 3 in a rotative direction thereof.
The fixed protrusions 4 to 6 are thermally deformed with a pin of a crimping jig in a state in which the fixed protrusions 4 to 6 are fitted into the supporting holes corresponding to the same, respectively, at the above-described three positions to squash into an umbrella shape, thereby crimping and fixing the lens 3 to the lens holding portion 2.
Patent Document 1: Japanese Published Unexamined Application No. 58-187907
Patent Document 2: Japanese Published Unexamined Application No. 2002-182089
However lens holding by fitting the three fixed protrusions into the supporting holes and by thermal crimping as described above is difficult to obtain position accuracy because the fixed protrusion as the reference and the fixed protrusion corresponding to the elongated hole are at a position opposed to each other at 120°, and is difficult to obtain a crimping condition because there are three kinds of fixed protrusions and hole shapes, respectively, which are subjected to the thermal crimping, and those three are different, thereby leading to a decrease in productivity.